The instructions for making the cells in the human body are encoded in deoxyribonucleic acid (DNA). DNA is a long, ladder-shaped molecule, in which each corresponding rung is made up of a pair of interlocking units, called bases, that are designated by the four letters in the DNA alphabet—A, T, G and C. ‘A’ always pairs or “bonds” with ‘T’, and ‘G’ always pairs or “bonds” with ‘C’. The sequence of these four letters that make up an individual's DNA is referred to as the individual's genome.